Hemodialysis is a process by which an artificial kidney replaces the function of a patient's kidney. Blood is removed from the patient's vascular system via suitable equipment such as an arterial needle, tube, or line, is passed through a dialyzer and is returned to the patient via a venous needle, tube, or line for normal circulation through the patient's vascular system. A majority of dialysis patients have an arteriovenous shunt implanted to create a location having a high blood flow that simplifies the withdrawal of blood through a line connected to the part of the shunt that is closer to the arterial side of the shunt and the return of purified blood through a line connected to the shunt downstream of the withdrawal site, closer to venous side of the shunt. In some cases the shunt clots or stenoses with the resulting reduction in blood flow necessitates surgery that is costly and invasive for the patient. If there is low blood flow in the shunt or any other problem with the venous outflow, some part of the freshly dialyzed blood from the venous return line flows directly to the arterial withdrawal line where it is again filtered. This access recirculation is a well-known problem during hemodialysis, and if such undesired direct recirculation is at a high enough level, some amount of blood is repeatedly refiltered so that the rest of the patient's blood is not sufficiently filtered for adequate dialysis.
One method of measuring shunt blood flow currently uses color coded duplex sonography. This is very expensive and involves operation by highly qualified professionals. Measurements are therefore made only rarely, so that the onset of reduced flow is not detected at a time when the problem can be corrected without surgery.
The standard test for undesired direct recirculation requires three blood samples while the patient is on dialysis. This method requires blood samples from the patient, time from the nurses, and high laboratory costs. Since dialysis patients generally have lower hematocrit than the normal population and are at greater risk from losing blood, this test is not very satisfactory.
Another technique for measuring access recirculation involves introducing an indicator, such as a saline solution, into the venous line and recording changes of blood properties due to the presence of the indicator in the arterial line. The problem with this technique is that the indicator can reenter the arterial line by two pathways: directly by the access shunt ("shunt recirculation") and by way of the patient s cardiopulmonary pathway. Existing technologies cannot separate shunt recirculation from cardiopulmonary recirculation (CPR). This can led to a false diagnosis of shunt recirculation in cases where only CPR is present.